The present invention relates to a novel optical design for laser encoder resolution extension with three-dimensional motion decoupling capability.
U.S. Pat. No. 5,896,200, issued Apr. 20, 1999 to Deming Shu issued to the present assignee and entitled xe2x80x9cOptical design for laser encoder resolution extension and angular measurement,xe2x80x9d discloses a laser Doppler encoder including a set of prisms mounted on a fixed base along with an additional prism, an end retroreflector and a laser/detector and a second set of prisms mounted on a moving/rotating base. The laser/detector generates a laser beam that is transmitted through the sets of prisms on the fixed and moving bases as well as the additional prism and the end retroreflector until the laser beam finally reaches a heterodyning detector that is housed coaxially inside the laser/detector. The laser beam is a frequency-stabilized laser beam such that the laser beam that is reflected back to the heterodyning detector is frequency-shifted by the movement of the moving base relative to the fixed base so that the amount of movement (either rotational or linear) of the moving base can be accurately determined. Moreover, the end retroreflector enables the laser Doppler encoder to be readily self-aligned such that the alignment time is substantially reduced and the three-dimensional optical path configuration results in a compact and integrated optical design that optimizes the system""s anti-vibration performance. The laser Doppler encoder can be used in conjunction with a high energy resolution monochromator for accurately determining the rotational movement of an arm in the monochromator or can be used in conjunction with a closed looped motion controller for providing feedback on the rotational displacement of the arm of the monochromator so that the arm can be accurately positioned.
The subject matter of the above-identified U.S. Pat. No. 5,896,200 is incorporated herein by reference.
While the above-identified U.S. Pat. No. 5,896,200 provides an improved optical design for laser encoder resolution extension and angular measurement, a need exists for a further enhanced optical design. It is desirable to provide such an enhanced optical design that enables three-dimensional measurements to be made with atomic scale resolution over a large two or three dimensional measuring range, rather than measurements in a single plane.
A principal object of the present invention is to provide a novel optical design for laser encoder resolution extension with three-dimensional motion decoupling capability.
In brief, an optical system is provided for laser encoder resolution extension with three-dimensional motion decoupling capability. The optical system includes a first prism mounted on a moving target, and a plurality of prisms, a retroreflector, a laser source, and a detector mounted on a fixed base. The moving target has three-dimensional linear motion freedom. The first prism on the moving target and the plurality of prisms and the retroreflector on the fixed base reflect a laser beam from the laser source to the detector define a three-dimensional optical path. The three-dimensional optical path provides multiple times optical resolution extension power for linear displacement measurement and encoding. This optic system is only sensible to the target motion on X direction and is substantially unaffected by movement in the Y and Z directions.
In accordance with features of the invention, instead of a typical single reflection on the moving target, the laser beam is reflected back and forth multiple times, such as twelve times or twenty-four times, between the fixed base and the moving target. The first prism mounted on the moving target is a larger prism than each of the plurality of prisms mounted on the fixed base. The detector is a heterodyning detector that is housed coaxially inside a frequency-stabilized laser source. The laser beam, which is reflected back to the heterodyning detector, is frequency-shifted by the movement of the moving target relative to the fixed base. With a laser Doppler displacement meter laser source and detector electronics, this optical path provides, for example, twelve times or twenty-four times optical resolution extension power for the linear displacement measurement and encoding.
In accordance with features of the invention, first and second optical design configurations are provided, each providing multiple self-aligned optical path design with three-dimensional motion decoupling capability. The plurality of prisms mounted on the fixed base in the first optical design configuration include at least one pair of prisms arranged substantially laterally aligned in spaced apart planes and at least a third laterally offset prism vertically spaced apart from the at least one pair of prisms. The retroreflector reflects the laser beam back to the laser source following the original optical path and finally reaching the detector that is arranged coaxially in a laser source housing. The retroreflector provides for a very practical self-alignment capability.
In accordance with features of the invention, the plurality of prisms mounted on the fixed base in the second optical design configuration include at least one pair of elongated prisms arranged substantially aligned in spaced apart planes and at least a third prism disposed near an upper plane between the at least one pair of elongated prisms.